Low energy plasma coating

ABSTRACT

A method of coating an aluminum alloy or magnesium alloy component, including cleaning and drying surfaces of the component to be coated; suspending a powdered coating material in a carrier gas and feeding the suspended powdered coating material through a plasma torch in a flowing gas; heating the coating material in the plasma torch to a molten or semi-molten state using a nominal power below 25 kW; and depositing the coating material with the plasma torch directly on the surfaces to be coated. The component may be made of a magnesium alloy containing at one or more of zinc, cerium and zirconium, or of an aluminum alloy containing one or more of magnesium, silicon, copper and chromium. The powder material may be made in majority of aluminum.

TECHNICAL FIELD

The application relates generally to coating and, more particularly, toa method suited for coating by low energy plasma gas turbine enginecomponents.

BACKGROUND OF THE ART

In a gas turbine engine, it is known to provide alloy components such asengine casings with an appropriate metal coating, for example forimproved resistance to corrosion, wear, heat and/or abrasion. Anintermediate bond coat is usually required between the alloy componentand the metal coating to provide for adequate bond strength. Theapplication of the bond coat however provides for additional costs andmanufacturing steps in the production of the coated component.

SUMMARY

In one aspect, there is provided a method of coating an aluminum alloyor magnesium alloy component, the method comprising: cleaning and dryingsurfaces of the component to be coated; suspending a powdered coatingmaterial in a carrier gas and feeding the suspended powdered coatingmaterial through a plasma torch in a flowing gas; heating the coatingmaterial in the plasma torch to a molten or semi-molten state using anominal power output below 25 KW; and depositing the coating materialwith the plasma torch directly on the surfaces to be coated.

In another aspect, there is provided a method of coating an aluminumalloy or magnesium alloy component, the method comprising: cleaning anddrying aluminum alloy or magnesium alloy surfaces of the component to becoated; suspending a coating material made in majority of aluminum in acarrier gas and feeding the suspended powdered coating material througha plasma torch in a flowing gas; heating the coating material in theplasma torch to a molten or semi-molten state; and depositing thecoating material with the plasma torch directly on the surfaces to becoated.

In a further aspect, there is provided a method of coating a magnesiumalloy component containing at least one material selected from the groupconsisting of zinc, cerium and zirconium, the method comprising:suspending an aluminum-based powdered coating material in a carrier gasand feeding the suspended powdered coating material through a plasmatorch in a flowing gas; heating the coating material in the plasma torchto a molten or semi-molten state; and depositing the coating materialwith the plasma torch directly on clean and dry surfaces of thecomponent to be coated, the plasma torch having a power output below 25KW.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 illustrates relative bond strength resistance values of aluminumcoatings on a surface of a magnesium alloy component obtained by low andhigh energy plasma with and without an intermediate bond coat; and

FIG. 3 is a diagram of a method of coating a component in accordancewith a particular embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication an air inlet 12 through which ambient air enters theengine, a compressor section 14 for pressurizing the air, a combustor 16in which the compressed air is mixed with fuel and ignited forgenerating an annular stream of hot combustion gases, and a turbinesection 18 for extracting energy from the combustion gases. The turbinesection 18 includes a power turbine driving an output shaft 22 which inturn drives a propeller 24 through a reduction gearbox 26. Although thegas turbine engine 10 has been shown here as a turboprop engine, it isunderstood that in other embodiments the engine 10 may be a turbofanengine, a turboshaft engine, an APU, etc.

A number of components of the engine 10 require protection fromcorrosion, wear, heat and/or abrasion. In a particular embodiment, thecomponent 28 being coated is an inlet case of the engine 10, whichdefines the engine inlet 12. Other components may also be coated asdescribed herein, for example the front housing, gearbox, pump, rearcase, front case, covers, etc.

One method of applying a coating is the use of plasma spray deposition,where a powdered coating material suspended in a suitable carrier gas isfed into a stream of flowing gas which is ionized and heated toextremely high temperatures by an electric arc. The coating particlesare heated to plasticity and carried onto the component in the resultinghigh velocity plasma stream.

Alloy components, for example made of magnesium or aluminum alloy, werepreviously protected by a metal coating, for example an aluminum-basedcoating, applied through high energy plasma spray deposition (e.g. usinga plasma gun with a power output of 40 KW), but an intermediate bondcoat, for example made of a nickel alloy, was required between thecomponent and the aluminum coating to obtain a desired bond strength.The inventor has found that by using a low energy plasma spraydeposition method (e.g. using a plasma gun with a power output of 16KW), the aluminum coating can be directly applied to the alloy component28 without the use of the intermediate bond coat, while still achievingthe desired bond strength.

In a particular embodiment, the component 28 is made of a magnesiumalloy containing at least one material selected from the groupconsisting of zinc, cerium and zirconium. In a particular embodiment,the alloy has a composition by weight including, in addition tomagnesium, from 3.5% to 5.0% of zinc (Zn), from 0.75% to 1.75% of totalrare earths (principally a mixture of cerium, lanthanum, neodymium andpraseodymium with a cerium of content of at least 45% of the total rareearths) and from 0.40% to 1.0% of zirconium (Zr).

In a particular embodiment, the magnesium alloy is AMS 4439 or similar,including from 3.5% to 5.0% of zinc (Zn), from 0.75% to 1.75% of totalrare earths (with a cerium of content of at least 45% of the total rareearths), from 0.40% to 1.0% of zirconium (Zr), up to 0.15% of manganese(Mn), up to 0.10% of copper (Cu), up to 0.01% of nickel (Ni) and up to0.3% total of other elements (with up to 0.1% per other element), thebalance being magnesium (Mg).

In another embodiment, the component 28 is made of an aluminum alloycontaining at least one material selected from the group consisting ofmagnesium, silicon, copper and chromium. In a particular embodiment, thealuminum alloy has composition by weight including, in addition toaluminum, from 0.8% to 1.2% of magnesium (Mg), from 0.4% to 0.8% ofsilicon (Si), from 0.15% to 0.40% of copper (Cu) and from 0.04% to 0.35%of chromium (Cr).

In a particular embodiment, the aluminum alloy is similar to AISI 6061,with a composition by weight including from 0.8% to 1.2% of magnesium(Mg), from 0.4% to 0.8% of silicon (Si), from 0.15% to 0.40% of copper(Cu), from 0.04% to 0.35% of chromium (Cr), up to 0.7% of iron (Fe), upto 0.25% of zinc (Zn), up to 0.15% of titanium (Ti) and up to 0.15% ofmanganese (Mn), the balance being aluminum (Al) and impurities.

The coating is provided in powder form and applied through low energyplasma spray deposition. In a particular embodiment, the coating powderis made in majority of aluminum. In a particular embodiment, the coatingpowder is made in majority of aluminum and includes, by weight, from 11%to 13% of silicon (Si).

In a particular embodiment, the coating powder has a composition byweight including from 11% to 13% of silicon (Si), up to 0.80% of iron(Fe), up to 0.30% of copper (Cu), up to 0.20% of zinc (Zn), up to 0.15%of manganese (Mn), up to 0.10% of magnesium (Mg) and up to 0.1% total ofother elements (with up to 0.05% per other element), the balance beingaluminum (Al).

Referring to FIG. 2, it has been found that the use of a low energyplasma deposition method to provide for a aluminum coating on amagnesium alloy component such as described above provides for similarbond strength values for both a coating applied directly to thecomponent (Low energy, Al) and a coating applied over an intermediatenickel-based bond coat (Low energy, Al/Ni), which is also similar and insome cases slightly superior than the bond strength of a similar coatingapplied to a similar component through high energy plasma depositionover an intermediate nickel-based bond coat (High energy, Al/Ni). It canbe also seen that the bond strength of the similar coating applieddirectly to the similar component, without the intermediate nickel-basedbong coat (High energy, Al) is significantly lower, e.g. more than halfthe bond strength obtained through the low energy plasma deposition. Theuse of a low energy plasma spray method thus allows for the aluminumcoating to have sufficiently good adherence to the surface of themagnesium alloy component that the intermediate bond coat can beomitted, which may help reduce the cost, time and complexity of thecoating application.

Similar results have also been observed for the application of analuminum-silicon coating as described above on an aluminum alloy castingcomponent.

In a particular embodiment, the bond strength achieved through the lowenergy plasma deposition is at least 3000 Psi; in a particularembodiment, the bond strength achieved is at least 7000 Psi.

In a particular embodiment and in reference to FIG. 3, the component 28is coated in accordance with the following.

Prior to the application of coating, the portions of the component 28which must not be coated, if any, are suitably masked, as set forth in100. The component 28 is also appropriately cleaned to be free fromdirt, grit, oil, grease and other foreign materials, as set forth in102. For example, the surfaces to be coated may be conditioned byblasting. In a particular embodiment, the blasting medium is aluminumoxide, zirconium oxide, or a mixture of these media; care is taken toavoid distortion or the embedding of abrasive particles due to excessiveblast pressure. Loose abrasive particles are removed from the componentbefore proceeding with the plasma spray coating.

The surfaces to be coated are dried, as set forth in 104. In aparticular embodiment, drying is achieved by preheating the component,for example, through control of the dwell time of the plasma spray torchimmediately prior to spraying.

As can be seen at 106, the blended powdered coating material issuspended in the carrier gas and fed into the stream of flowing gaswhich is ionized and heated by the electric arc, thus heating thecoating material to a molten or semi-molten state, as set forth in 108.In a particular embodiment, the carrier gas and the flowing gas areargon, helium, a mixture of argon and helium, or a mixture of either ofthese gases with up to 20% hydrogen by volume.

In a particular embodiment, both the carrier gas and the primary flowinggas are argon, and hydrogen is provided as a secondary flowing gas, withthe flow of primary gas being about 40 L/min, the flow of secondary gasbeing about 1 L/min, and the flow of carrier gas being about 4 L/min,with the powder feed being from 8 g/min to 12 g/min. In a particularembodiment, the flow of primary gas within a range of 40 L/min±10%, theflow of secondary gas is within a range of 1 L/min±10%, and/or the flowof carrier gas is within a range of 4 L/min±10%.

The heated coating particles are deposited directly on the surface ofthe component through the high velocity plasma stream, as set forth in110. In a particular embodiment, the plasticized coating particles areaccelerated to a speed in the range of 200 to 300 m/s.

In a particular embodiment, the low energy plasma spray depositionmaintains the temperature of the component during spraying below 400° F.(204° C.).

In a particular embodiment, the plasma spray deposition is performed adistance of 1 inch from the surface of the component 28 to be coated.

In a particular embodiment, “low energy plasma deposition” is defined asbeing performed using a plasma torch with a nominal power of below 25kW, preferably at most 16 kW. In a particular embodiment, the low energyplasma deposition is performed using a SM-F210 plasma gun manufacturedby Sulzer Metco. However, it is understood that any other suitable lowenergy plasma torch may be used.

To form the coating, the head of the plasma torch may pass over thesurface of the component 28. The number of passes required is a functionof the thickness of the coating to be applied. The torch may be heldstationary to form a thick deposit over the area to be coated. It ishowever desirable to limit the thickness per pass in order to avoid aquick build up of residual stresses and unwanted debonding betweendeposited layers.

The component 28 is thus coated by applying the coating directly to itssurface, i.e. without the use of an intermediate bond coat.

The application method may also be used for the repair of the alloycomponents using an aluminum-containing repair material. When repairingan alloy component of, for example, a gas turbine engine, corrosion pitsand/or damaged areas are mechanically removed, for example throughgrinding, machining or other applicable techniques. The resultingsurface may optionally be grit blasted prior to depositing thealuminum-containing repair material using the above described method. Inat least an embodiment, the metal containing repair material comprises amaterial which has a composition that includes more that 50% by weightof aluminum.

EXAMPLE 1

Parameters of a high energy and low energy plasma projections accordingto a particular embodiment are set forth below:

Max. Primary Secondary Powder Coat- gas flow gas flow Nominal Spray feeding (Argon) (H₂) Amp. Power distance rate Temp. (SCFH) (SCFH) (A) (kW)(inch) (g/min) (° C.) Low 84.75 2.12 320 10.4 1 3 120 energy High 96 20500 73 5 75.6 N/A energy

EXAMPLE 2

A magnesium alloy component made of AMS 4439 was coated by low energyplasma using a powder having a composition by weight including from 11%to 13% of silicon, up to 0.80% of iron, up to 0.30% of copper, up to0.20% of zinc, up to 0.15% of manganese, up to 0.10% of magnesium and upto 0.1% total of other elements (with up to 0.05% per other element),the balance being aluminum. The particle size distribution of thecoating powder was in conformity with (where + indicates retained onsieve and − indicates passing sieve):

% by weight ASTM Sieve minimum maximum +140 — 1 +170 — 7 −325 — 10 +32590 —

The coating was performed using a SM-F210 Internal Plasma spray gun bySulzer Metco as the low energy plasma torch, having a power output of 16kW.

The coating was sprayed using the following parameters:

Primary gas flow: Argon, 40 L/min

Secondary gas flow: Hydrogen, 1 L/min

Carrier gas flow: Argon, 4 L/min

Power: 300 Amps

Spray distance: 1 inch

Power feed rate: 8 to 12 g/min

The surface of the magnesium alloy component was subjected to a regulargrit blast prior to the coating.

The bond strength of the coated magnesium alloy component thus obtainedwas 7000 Psi, the coating hardness was 79.6 HV, and the average coatingthickness was 0.016 inch.

A corrosion immersion test was performed by immersing samples in asolution of 3.5% NaCl in deionized water. A weight loss of 19% wasobserved after an immersion of 48 hours, with an average thinning rateof 1% per hour. A salt spray test was also performed using a spraysolution of 5% NaCl in deionized water. A weight loss of 3% was observedafter 46 hours in the spray.

As such, despite the absence of the bond coat, the corrosion resistancewas shown to be within acceptable values.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Modifications which fall within the scope of the present invention willbe apparent to those skilled in the art, in light of a review of thisdisclosure, and such modifications are intended to fall within theappended claims.

1. A method of coating an aluminum alloy or magnesium alloy component,the method comprising: cleaning and drying surfaces of the component tobe coated; suspending a powdered coating material in a carrier gas andfeeding the suspended powdered coating material through a plasma torchin a flowing gas; heating the coating material in the plasma torch to amolten or semi-molten state using a nominal power output below 25 kW;and depositing the coating material with the plasma torch directly onthe surfaces to be coated.
 2. The method as defined in claim 1,depositing the coating material with the plasma torch is performed whilemaintaining a temperature of the surfaces to be coated below 400° C. 3.The method as defined in claim 1, wherein the plasma torch has a poweroutput of at most 16 KW.
 4. The method as defined in claim 1, whereindepositing the coating material with the plasma torch directly on thesurfaces to be coated includes depositing the coating material directlyon the surfaces made of a magnesium alloy containing at least onematerial selected from the group consisting of zinc, cerium andzirconium.
 5. The method as defined in claim 3, wherein the magnesiumalloy has a composition by weight including from 3.5% to 5.0% of zinc,from 0.75% to 1.75% of total rare earths with a cerium of content of atleast 45% of the total rare earths, and from 0.40% to 1.0% of zirconium.6. The method as defined in claim 1, wherein depositing the coatingmaterial with the plasma torch directly on the surfaces to be coatedincludes depositing the coating material directly on the surfaces madeof an aluminum alloy containing at least one material selected from thegroup consisting of magnesium, silicon, copper and chromium.
 7. Themethod as defined in claim 6, wherein the aluminum alloy has acomposition by weight including from 0.8% to 1.2% of magnesium, from0.4% to 0.8% of silicon, from 0.15% to 0.40% of copper, and from 0.04%to 0.35% of chromium.
 8. The method as defined in claim 1, whereinsuspending the blended powdered coating material includes suspending apowder material made in majority of aluminum.
 9. The method as definedin claim 8, wherein the powder material includes, by weight, from 11% to13% of silicon.
 10. The method as defined in claim 1, wherein depositingthe coating material includes accelerating the molten or semi-moltencoating material to a speed of from 200 to 300 m/s.
 11. The method asdefined in claim 1, wherein depositing the coating material includesprojecting the coating material at a rate of from 8 to 12 g/min.
 12. Themethod as defined in claim 1, wherein suspending the powdered coatingmaterial in a carrier gas includes suspending the powdered coatingmaterial in argon, and feeding the suspended powdered coating materialthrough a plasma torch in a flowing gas includes feeding the suspendedpowered coating material in a primary gas flow of argon with a secondarygas flow of hydrogen smaller than the primary gas flow.
 13. A method ofcoating an aluminum alloy or magnesium alloy component, the methodcomprising: cleaning and drying aluminum alloy or magnesium alloysurfaces of the component to be coated; suspending a coating materialmade in majority of aluminum in a carrier gas and feeding the suspendedpowdered coating material through a plasma torch in a flowing gas;heating the coating material in the plasma torch to a molten orsemi-molten state; and depositing the coating material with the plasmatorch directly on the surfaces to be coated.
 14. The method as definedin claim 13, wherein depositing the coating material includes depositingthe coating material directly on the surfaces made of magnesium alloycontaining at least one material selected from the group consisting ofzinc, cerium and zirconium.
 15. The method as defined in claim 14,wherein the magnesium alloy has a composition by weight including from3.5% to 5.0% zinc, from 0.75% to 1.75% of total rare earths with acerium of content of at least 45% of the total rare earths, and from0.40% to 1.0% of zirconium.
 16. The method as defined in claim 13,wherein depositing the coating material includes depositing the coatingmaterial directly on the surfaces made of aluminum alloy containing atleast one material selected from the group consisting of magnesium,silicon, copper and chromium.
 17. The method as defined in claim 16,wherein the aluminum alloy has a composition by weight including from0.8% to 1.2% magnesium, from 0.4% to 0.8% silicon, from 0.15% to 0.40%copper, and from 0.04% to 0.35% chromium.
 18. The method as defined inclaim 13, wherein suspending the blended powdered coating materialincludes suspending a powder material made in majority of aluminum. 19.The method as defined in claim 13, wherein the plasma torch has a poweroutput below 25 KW.
 20. A method of coating a magnesium alloy componentcontaining at least one material selected from the group consisting ofzinc, cerium and zirconium, the method comprising: suspending analuminum-based powdered coating material in a carrier gas and feedingthe suspended powdered coating material through a plasma torch in aflowing gas; heating the coating material in the plasma torch to amolten or semi-molten state; and depositing the coating material withthe plasma torch directly on clean and dry surfaces of the component tobe coated, the plasma torch having a power output below 25 KW.